Cook the potatoes: Place the cubed potatoes in a large pot of salted water. Bring to a boil and cook until fork-tender, about 15 minutes. Drain and return them to the pot.
2 pounds Yukon Gold potatoes
Sauté the leeks and kale: In a large skillet over medium heat, melt 2 tablespoons of butter. Add the sliced leeks and cook for 3–4 minutes until softened. Add the garlic and chopped kale, cooking for another 2–3 minutes until the kale wilts. Remove from heat.
1 leek, 2 cloves garlic, 5 ounces kale
Mash the potatoes: Warm the heavy cream and remaining 2 tablespoons of butter in a small saucepan or microwave until melted. Mash the potatoes, then stir in the warm butter and cream mixture.
½ cup heavy cream
Combine everything: Fold the sautéed leeks and kale into the mashed potatoes. Season with salt and pepper to taste.
1 teaspoon salt, ½ teaspoon ground black pepper
Serve: Transfer to a serving dish, garnish with more butter chopped green onions, and enjoy warm!